In a non-contact magnetostrictive torque detecting device, when a to-be-measured shaft is manufactured using steel for machine structural use, the magnetostrictive phenomenon does not occur so conspicuously that an accurate detection of the torque is difficult to achieve. Accordingly, the torque detecting device has been suggested, in which a special alloy, for example, an Fe—Al alloy is used for the to-be-measured shaft. See, for example, the Japanese Patent No. 2592491. However, since the Fe—Al alloy is hard, but fragile, the mechanical processability is reduced considerably. In a case of a to-be-measured shaft having notches, the torsional fatigue strength and the impact value are also reduced significantly. For this reason, application of this alloy as a material for a structural component has been difficult to achieve.
As an alternative measure, attempts have been made to manufacture a shaft with the use of the Fe—Al alloy only on a surface region of a to-be-detected portion of the to-be-measured shaft while steel for machinery structural use is jointed to a portion of the shaft where strength and toughness are required. In such case, the jointed portion lacks a reliability and is costly to make. For this reason, various shafts have been suggested, in which the Fe—Al alloy is welded to a necessary portion of a surface region of the to-be-measured shaft, but all of them involve the following problems.
By way of example, where a high magnetostrictive material such as an Fe—Al alloy is overlaid on a surface region of the to-be-measured shaft made of steel for machine structural use, which is a to-be-detected portion of the to-be-measured shaft, by the use of an overlay welding technique so that such to-be-detected portion can be used as a target area subject to torque detection such as disclosed in the Japanese Patent No. 2765340, a problem has been encountered with that since the Fe—Al alloy is fragile, cracking and/or exfoliation tend to occur during the overlay welding. In an attempt to eliminate such problem and particularly to avoid the cracking, it has been suggested to effect the overlay welding to form the overlaid Fe—Al alloy layer of a thickness equal to or smaller than 3 mm. However, since when the overlay welding or spray coating (such as employed in the Japanese Patent No. 2765340) is effected, pores tend to occur, which constitute a cause of premature cracking brought about by a cyclically loaded torque and also that of deviation in magnetostrictive characteristic. This deviation in magnetostrictive characteristic will in turn leads to considerable reduction in detecting accuracy.